Application of a new method for determining digestible reactive lysine to variably heated protein sources

The study evaluated a recently developed bioassay for determining digestible reactive lysine when applied to skim milk powder and field peas, which had been exposed to varying degrees of heat treatment. Semi-synthetic corn starch-based diets containing the respective material as the sole source of protein were formulated and fed to growing rats. Chromic oxide was included as an indigestible marker. Digesta were sampled from the terminal ileum post-mortem, and diet and digesta were analyzed for amino acid content (including reactive lysine, which was determined following guanidination). True ileal, reactive lysine and total lysine digestibilities were calculated. For the heated skim mills. powder, both total and reactive lysine digestibility decreased from 97% and 100% (unheated) to 44% and 85%, respectively (121 degrees C for 10 min). A similar trend was observed for the heated peas where total and reactive lysine digestibility decreased from 83% and 88% (unheated) to 43% and 67%, respectively, in the maximally heated peas. In all cases, total lysine digestibility significantly underestimated reactive lysine digestibility. In contrast, digestible total lysine (conventional method) overestimated digestible reactive lysine for all the skim milk powder samples and some of the pea samples. This overestimation was almost double for the maximally heated skim milk powder where digestible total lysine was 11.2 g kg(-1) and digestible reactive lysine was 5.7 g kg(-1).